The present invention relates generally to the manufacture of articles by sintering techniques and more specifically to a powder compression tool for forming a work piece herein termed a compact, which is then placed in a sintering furnace.
In general terms, sintering consists of compressing metal powder, generally a steel powder, to obtain a compact of definitive shape. This compact whose shape is maintained only by cohesion of the powder, is then passed through a furnace at a sintering temperature below the melting temperature, but sufficient for the powder particles to join.
After sintering, the product will typically exhibit a final density which approaches, but does not equal the density of the metal in question. In the case of steel powder, it is possible to achieve final densities on the order of 7.4-7.5 g/cc, using the conventional pressing and sintering techniques described below, whereas the density of steel itself is on the order of 7.8-7.9. For ease of reference, this will be referred to as the maximum density.
It is an object of the present invention to provide a modified pressing process and apparatus capable of operation to yield sintered products having a final sintered density which more nearly approaches the maximum density of the material, and in the case of steel, a final sintered density of over 7.5. According to the present invention, this is achieved by a single press, single sinter process in which a metal powder mix containing from about 0.3 to 0.5 weight % of a solid lubricant is pressed in a single step in a die having a working clearance of at least 45 xcexcm under a pressure of at least 800 MPa to form a compact for subsequent sintering.
For better understanding of the basic technology, conventional powder compression processes will now be described with the aid of FIGS. 1A, 1B and 1C.
FIGS. 1A to 1C illustrate the operation of a powder compression tool. The tool includes a die 10 with a cavity 12 arranged through it. This cavity 12 defines the shape or profile of the desired compact, including features such as a smooth surface, an indentation, or other characteristic. Die 10 co-operates with an upper punch 14 and a lower punch 15 which penetrate through both ends of the cavity 12.
In FIG. 1A, the cavity 12 is filled with metal powder flush with the upper surface of die 10. Lower punch 15 is at a specific position determined by the volume of powder required to obtain the desired height and density of the final produce Once cavity 12 is filled with powder, upper punch 14 is lowered.
In FIG. 1B, upper punch 14 reaches an end position determined by the pressure applied to both punches. A compact 17 of desired shape is then obtained in cavity 12, formed of powder particles sufficiently cohered together to allow it to be handled and carried to a sintering furnace (not shown).
In FIG. 1C, upper punch 14 is withdrawn while lower punch 15 is raised to eject compact 17 from the cavity 12. The compact is then carried to the sintering furnace. To eject Impact 17, instead of raising the punch 15, die 10 could be lowered. It will be appreciated that various options are possible.
As illustrated in FIGS. 1A and 1B, the volume of the powder decreases considerably on application of pressure. For conventional pressures, on the order of 700 MPa. the volume decreases by a factor 2.3 to 2.5. This decrease in volume is accompanied by rubbing of the powder against the wails of the cavity 12 over the length of travel of the punch. It is thus essential to lubricate the walls of the cavity 12 to minimise friction.
Lubrication of the walls of the cavity 12 being impractical in production, it is preferred to include the lubricant in the metal powder. For the powder to be able to properly flow to fill up the cavity, the lubricant also comes as a powder.
Of course, lubrication also facilitates ejection of the compact 17, without damage.
The proportion of lubricant commonly used in the metal powder is from 0.6 to 0.8% in weight. However, the lubricant is about eight times less dense that the metal powder, and occupies an incompressible volume which cannot be replaced by metal during the compression. As a result, especially upon elimination of the lubricant while sintering. the obtained compacts are porous and have a mechanical strength which is substantially lower than that of pure metal.
Thus in practical terms, conventional pressing and sintering processes can yield products with a final density (in the case of steel) of up to 7.5. More typically, using a pressure of 700 MPa and 0.8% lubricant, the final density is only around 7.15. In theory, higher pressures would tend to increase the final density, but in practice, pressures exceeding about 800 MPa have been observed to result in rapid tool damage, even though the tool itself is, in isolation, capable of withstanding more than 2000 MPa.
It is appropriate to mention that final sintered density is much more significant than the so-called xe2x80x9ctheoretical maximum densityxe2x80x9d of the compact, including lubricant, before sintering. Reducing the lubricant quantity may make it possible to achieve a higher percentage of the maximum theoretical density of a particular metal powder/lubricant mixture, but even values such as 96% of maximum theoretical density correspond only to a final sintered density of 7.15 in the case of steel powder containing 0.8% lubricant.
A final density, in the case of steel powder, of around 7.15 thus is typical of that obtained through a conventional single press/single sinter process, in which a single powder compression step is performed, at about 700 MPa, followed by sintering to obtain the final product.
To obtain sintered compacts with higher densities, a double press/double sinter process can be used, in which, after compression under the above-mentioned conditions, the compact undergoes a pre-sintering treatment to vaporise the lubricant, so as to empty the pores that it occupies. The compact is then submitted, before a final sintering, to a second compression during which the material, not yet generally integral, tends, through plastic deformation, to occupy the empty pores. With such a process, however, final densities above 7.5 cannot be achieved. Further, such a two-stage process is more expensive to implement than a single press/single sinter process.
There is also a warm forming process in which the die and powder are heated to about 100-150xc2x0 C. to liquefy the lubricant which then escapes by draining from the pores. The maximum densities obtained are on the order of 7.4 (in the case of steel) and the process is also expensive to implement.
A further object of the present invention is to provide a compression tool which can more successfully withstand operation at higher than normal pressures.
Yet another object of the present invention is to provide such a tool which enable compacts of particularly high final density to be obtained through a single pressing process.
In conventional compression tools, the clearance between punches and dies has always been made as small as possible. This is to avoid or at least minimise extension of powder through the clearance, as well as the formation of moulding flash, generally referred to as xe2x80x9cbeardsxe2x80x9d. The clearance commonly found in typical tools ranges from 10 to 20 xcexcm.
FIG. 2 illustrates on an enlarged scale the clearance in the tool and the deformation which take place during a compression operation. The nominal diameters of moving punch 14 and of cavity 12 of the die are indicated in dotted lines. During compression, punch 14 tends to undergo barrel deformation. At a certain pressure level, the punch comes into contact with the die along As entire circumference while still moving. The resulting friction increases as punch 14 comes closer to its final position and the deformation also increases.
If the friction were uniform over the punch circumference, the tool would be able to better withstand high pressure. However, in practice, punch 14 always rubs more against one side than against the other, which causes a high bending stress in the punch and even in the die. The compression tool, which is designed primarily for hardness, poorly withstands bending stress and prematurely deteriorates if the pressure exceeds 800 MPa.
Of course. the friction of punch 14 against die 10 may also damage the surface finish of the cavity 12, making the subsequent ejection of the compact 17 more difficult and affecting in turn its surface finish and that of components subsequently pressed in the die.
As shown in FIG. 2, the compact 17 itself also tends to undergo barrel deformation when under compression. pushing against the side walls of the cavity 12. When the punch is withdrawn, the compact 17 and the walls of cavity 12 may, if excessive force has been applied, undergo permanent deformation, making the ejection of the compact more difficult. This ejection is normally facilitated by the presence of a sufficient amount of lubricant, of course.
To avoid or at least minimise the above-mentioned problems, the present invention provides an increased clearance between the elements of the tool, especially between the moving punch and the die, so that this clearance is not affected by any deformation of the elements during the compression operation. The presence of a clearance may tend to accentuate the generation of beards on the edges of the produced compacts, but such beards only affects, for the most part the aesthetic appearance of the compacts. The increased clearance is preferably not greater than the mean grain size of the powder, or else the powder grains will tend to jam together in the gap, thereby increasing friction as well as causing excessive loss of powder, in an extreme case.
The article by G Bockstiegel et al: xe2x80x9cThe influence of lubrication, die material and tool design upon die wear in the compacting of iron powdersxe2x80x9d, Modem Developments in Powder Metallurgy, Proceedings of the 1970 International Powder Metallurgy Conference, vol. 4, 1971, New York, London, describes experiments made with punch/die clearances of 5, 10, 25 and 45 xcexcm, and concludes that the use of large clearances is detrimental in terms of tool wear.
In contrast the present invention uses large clearances, in particular greater than 45 xcexcm.
According to one embodiment of the present invention, the elements of the tool are arranged to form a compact having one face flush with a surface of the die.
According to another embodiment of the present invention, the tool includes a second punch (15, 14) co-operating with the cavity (12) from the side opposite to the point of entry of the first punch, the second punch, during compression, being arranged to seal the cavity at or in the vicinity of the die surface, the first punch being used to eject the compact at the end of compression.
According to another embodiment of the present invention, the first punch (15) includes axially protruding edge portions which serve to form recessed edge regions on the compact these edge regions serving to accommodate to a significant extent any beards formed.
According to a further embodiment of the present invention, the walls of the cavity are coated with a material having a low friction coefficient relative to the powder and which is able to withstand repeated use.
According to a preferred embodiment of the present invention, the coating is of a diamond-like carbon material.
According to a preferred embodiment of the present invention, less than 0.5, or more preferably, less than 0.4 weight % of lubricant is included in the powder to be moulded into a compact.
According to a particularly preferred embodiment of the present invention, the powder includes about 0.3% weight of lubricant when the die cavity walls are coated as mentioned above.
It is preferred that the green density of the compact prior to sintering is at least 7.4 g/cc,
It has been found that the present invention can in the case of steel powder, achieve, by a cold, single pressing/sintering of a mixture of metal powders and significantly reduced amount of lubricant, a final density of at least 7.5.